To date locomotives have been equipped with mechanical or electromechanical speedometers which essentially detect wheel revolutions per unit time and display this information in terms of miles per hour (mph), for example. Such speedometers have serious problems due to the inaccuracy resulting from the wear of the wheels over a period of time, because of wheel slip particularly at low speeds which increases wear, and finally because of the inherent inaccuracy of such speedometers at low speeds particularly below one mph.
The need for accurate speed measurements, particularly at low speeds has increased as the result of introducing unit trains. The reason being that such trains are frequently loaded with coal, for example, by pulling empty cars at low speeds substantially in the vicinity of one mile per hour under a loading hopper. Any significant variation in speed from the desired speed can result in the cars being under or overloaded.
The application of Doppler radar techniques to measure the speed of automobiles, for example, is well known but such applications generally rely on movement of the vehicle whose speed is to be measured along the antenna beam axis, or boresight. Applying such Doppler radar techniques to measuring the speed of a locomotive is not practical since oncoming or passing trains on adjacent tracks would be within the beam of the transmitted microwave energy and would produce erroneous readings. To prevent moving objects from entering the beam of the radar antenna mounted on such locomotive, the antenna is mounted on the underframe of the locomotive pointed downwardly at the roadbed at a substantial angle and preferably pointing backwards with respect to the front of the locomotive.
With the axis of the transmitted beam of RF energy substantially coinciding with the direction of motion of the target or vehicle, essentially a single Doppler frequency is generated, or at least the amplitude of the single frequency so predominates that it is easily processed as a single frequency which is linearly proportional to the speed of the target. However, if the antenna which transmits and receives the RF energy is mounted at a significant angle with respect to the reflecting surface, the Doppler signals produced do not have a single frequency that substantially predominates but rather produces a spectrum, or distribution, of signals of many frequencies and amplitudes. The reason is that the transmitted beam of electromagnetic energy has a finite beam width so that the beam intersects a substantial area of the roadbed. The spectrum of the Doppler signals so produced differs from the ideal distribution of such signals in part because the reflective characteristics of the roadbed within this area also vary substantially so that the amplitude of the signals vary with time. The spectrum also differs from the ideal because of the unusual geometry of the reflective surface due to grade crossings, guard rails, and to changes in the distance between the roadbed and the antenna mounted on the locomotive, for example.
Attempts to use Doppler radar techniques to measure the speed of locomotives have not heretofore been successful to the degree of precision desired over the desired ranges of speeds because the prior art methods of processing the Doppler signals produced by various types of roadbeds under all types of weather and track conditions have heretofore sought to identify and process the single Doppler frequency, the boresight frequency, which would correspond to that produced by reflecting the transmitted RF energy from that point of the roadbed where the axis of the antenna, its boresight, intersects the roadbed.